First, the Vietnamese were deeply influenced by Chinese values with regard to Han characters. Since Hanji was regarded highly as the only official orthography in China, which was the suzerain of Vietnam, the Vietnamese people had no choice but to follow this traditional value assignment. As a consequence, the Vietnamese rulers in all dynasties, except a few short-lived strongly anti-Chinese rulers, such as Ho Quy Ly (1400-1407) and Quang Trung (1788-1792), had to recognize Han characters as the institutional writing criteria.

How was Vietnam able to successfully replace Han characters and Chu Nom with romanized Chu Quoc Ngu? I would attribute this success to two crucial factors: 1) internal factors of social demand for literacy and anti-feudal hierarchy, and 2) external factors of political interaction between Vietnam and China in the international sphere during the first half of the 20th century. These two crucial points also apply to other cases of language and orthographic reform in the Han sphere.

Chiung, Wi-vun T. 2003. Learning Efficiencies for Different Orthographies: A Comparative Study of Han Characters and Vietnamese Romanization. PhD dissertation: University of Texas at Arlington.